植入不良/造血延迟是造血干细胞移植后严重并发症，影响移植受者的生存率与生存质量。骨髓血管niche是造血干细胞归巢的门户，并调控造血干细胞增殖与分化，移植后血管niche损伤是造血干细胞植入失败或造血延迟的主要病理基础。. 内皮祖细胞（EPC）是内皮细胞的前体细胞。我们前期研究发现EPCs可促进骨髓损伤血管niche修复，促进早期造血重建，但远期疗效欠佳；有研究显示，EPCs形成有效血管腔需周细胞支持；骨髓中的MSCs是血管niche的重要周细胞，同时MSCs还是重度GVHD治疗的有效策略之一。为此，本研究则拟在前期工作基础上，以EPCs和MSCs为工具，探索骨髓血管niche修复，促进造血重建、防治移植后植入不良/造血延迟的作用，并进一步探索其修复损伤血管龛niche的机理。如获成功，不仅可为临床造血干细胞移植并发症的防治提供新策略，还可为其他血管损伤性疾病的防治研究提供新思路。

Engraftment failure and delayed hematopoiesis are severe complications after hematopoietic stem cell transplantation, affecting patients’ survival and life quality. Bone marrow vascular niche is the place where hematopoietic stem cells home to and regulates the proliferation and differentiation of hematopoietic stem cells. Vascular niche injury after transplantation is the major pathological basis for hematopoietic stem cells engraftment failure and delayed hematopoiesis..Endothelial progenitor cells (EPCs) are the precursor cells of endothelial cells. We have previously demonstrated that EPCs could promote the repair of damaged bone marrow vascular niche and early hematopoietic reconstitution. However, the longer effect is not satisfactory. Some studies showed that EPCs-mediated formation of effective vascular cavity requires the support of stromal cells. Bone marrow mesenchymal stem cells (MSCs) are not only important stromal cells of vascular niche, but also one of the effective strategies to treat severe graft-versus-host disease (GVHD). Therefore, based on previous works, using EPCs and MSCs as tools, this project aims to investigate their role in the repair of bone marrow vascular niche, promotion of hematopoietic reconstitution, as well as prevention of engraftment failure and delayed hematopoiesis after transplantation. In addition, this project further studies the mechanism of EPCs/MSc-mediated repair of damaged vascular niche. If succeed, it not only provide novel strategies for the prevention of hematopoietic stem cell transplantation-associated complications in clinic, but also provide theory for studies on the prevention of other vascular damage-associated diseases.